When you connect a portable generator to your house, you’re supposed to open the main breaker to disconnect your house from the grid. Supposedly this is to protect utility workers who may be working to repair utility lines: the idea is that if your generator is connected to the grid, it may subject those workers to hazardous voltages.
Obviously a person seeking to power their house with a portable generator should stick to that policy. But I’m wondering whether a portable generator would even run properly if the house’s main breaker is closed. If the house’s main breaker is open, then the generator is just trying to power your house, and the generator should have enough capacity for satisfactory operation. But if the house’s main breaker is closed, then the generator is trying to power the whole damn neighborhood (assuming the break in the grid isn’t between your house and the neighborhood’s distribution transformer). In that situation, wouldn’t the excessive current flow cause the generator’s own circuit breaker to trip out?
If the current draw on the generator is too large, it will shut down. (Or explode, depending on how well it’s designed.) But imagine a situation where a storm knocks down a line to just your house or a small number of houses. Or a situation where automatic reclosers near your house have isolated your neighborhood from the rest of the grid. And suppose it happens at night, when your neighbor’s have most of their stuff turned off, anyway. In such cases it is very easy for a home generator to end up supplying sufficient power to the localized grid to be dangerous.
I connect my portable generator to the house whenever the power goes out. I simply turn off the main circuit breaker, and then connect the generator to a 50 A receptacle. Works great, but not exactly “legal.”
I haven’t given much thought to what would happen if I were to close the main circuit breaker with the generator also connected (and the utility power also out). If the power was out due to an open circuit on the high voltage line, then I suppose I could power other homes, though it would be overloaded. And the voltage to other homes may be low due to being stepped up and then stepped down (two transformers). If the power was restored, my generator would probably melt down.
If a generator is attached to too much load, it’ll slow or stop, depending on the power of the motor that’s driving it, and the voltage will decrease. This might cause damage to the generator, the things it’s trying to power, or both.
If the generator is still attached when the grid comes back online, then you’re probably going to get a phase mismatch, which will result in your generator being very quickly and emphatically knocked into phase, which will probably damage it.
If it’s just a generator, that’s true. But all generators also have control circuitry that will shut down the generator to prevent damage if there’s too much draw, as well as breakers to deal with a dead short or sudden spike.
In a city, the last transformer (on a pole or a pad near your house) typically connects 2-4 houses. Those are all connected together, on the 240 volt downside of the transformer.
So a few houses (likely your next-door neighbors) will get power from your generator without going thru any transformers. So they would get power just like your house. (Of course, it’s also trying to send power thru that transformer and out to the rest of the grid, so the overload issues are still a concern.)
In order for the portable generator to stay on line it would have to sync up with the grid. Every generator on the grid is in sync, that is, it’s phase(s) voltage(s) are at the identical level of every other generator. (OK not identical, but the differentiation from generator to generator is quite small)
If two generator systems are joined together out of sync the stronger system will “pull” the weaker system towards synchronization. The result is the weaker system (in the op’s question that system is a home generator) will either fall into synchronization or it will fail and shut down.
Whenever a utility puts a new generator on line it will use equipment that compares the rotation of the generator with the grid and adjust that generator to a close approximation speed and then attaches it.
So to answer the op’s question. The home generator will do one of three things.
If it has a synchronization feature it will adjust and lock in to the grid
If it doesn’t have synchronization but is either built well enough to take the shock or lucks into a close approximation of the synchronous cycle it will moan and groan a little and then lock in.
If it doesn’t have a synchronization feature or the conditions stated in (2) it will break down. Quite possibly in a very spectacular fashion
On second read it looks like the op was referring to a grid outage so synchronization isn’t an issue.
If the main breaker isn’t open then the rest of the neighborhood will act as a load on the home generator and it will probably fail because it lacks the capacity to output enough power to satisfy the neighborhood load.
If the grid is not powered the generator will power it to it’s limited ability till it trips out.
If the generator is feeding the grid it is creating a hazard for line workers.
If the grid is powered while the generator is feeding the grid, the options range from the generator explodes to it continues to power the grid at your expense. The deciding factor is how far out of phase the generator is from the grid.
It is standard practice for utility workers to ground both ends of the wire (wires) they will be working on. Utility workers will never touch a line unless they have confirmation that it is grounded /not-live.
Having said that, there are multiple levels of safety and the household generator should not be connected to the grid.
I have read of fires resulting from generators connected to the grid when the grid is repaired (and if you are connecting to your utility pole but it is not connected to a power plant, I don’t think you’re actually connected to the grid anyway). Apparently it’s not unknown for generators to thrash around and explode and even ignite gas tanks in vehicles that happen to be nearby.
By the way, you can use an induction motor as a generator if you parallel capacitors with it. This isn’t quite as foolproof as typical synchronous home portable generators, but it has the advantage that it’s perfectly happy getting connected to the grid. Giving it the gas has only a small effect on its rotational speed (having to do with the degree of slip, which in turn has a lot to do with its efficiency). The effect giving it gas does have is to turn your electric meter backwards, helping the whole grid in a pitifully tiny way, and lowering your electric bill by much less than it costs to do this.
In the olden days, people would bring generators online using incandescent lamps between generator and mains to adjust the speed until it was running in sync, and then hitting the switch. There would be a dramatic show of flashing lights that would get slower and slower until it was ready.
I have synced generators using sync lights and a sync scopes. There is no flashing of lights. The lights go from out to bright and back to out. You adjust the governor so the generator coming on light is just a little faster than the bus. The lights should be slowly going from bright to out. Just before the lights go out start pulling the breaker handle closed. This was on merchant ships.
I know of one case where the engineering crew tried to take a short cut and parallel with PG&E. The Midshipman on the control platform got excited and pulled the breaker when the lights were the brightest. 180 degrees out of phase. I understand it blew every breaker and fuse between Vallejo and San Francisco.
I have also synced generators using sync lights (and also with a sychroscope). The farther the frequencies are off from each other, the faster the lights flash. I don’t know if I would describe it as “dramatic”, but you can have them flashing pretty quickly until you get the frequencies adjusted. Once you get them pretty close, then the lights will slowly fade in and out, like you describe.
For those that aren’t familiar with the process, you want the lights to roll pretty slowly so that you have plenty of time to activate the breaker while they are still in phase.
And again, for those not familiar with the process, this is a good example of how NOT to do it. When the lights are on full, that is literally the worst possible time to throw the switch.
Yikes! :eek:
The worst I’ve seen was someone throwing the breaker when two of the phase lines were reversed. The breaker tripped, but I still thought the generator was going to come apart. I was surprised that no permanent damage was done.
See, that’s the thing. If all the breakers and fuses between Vallejo and San Francisco may blow if you pick the wrong moment, the flashing is dramatic. And you’re never really 100% sure the generator’s not going to come apart until it didn’t.
An ice storm several years ago took out power for two weeks. I disconnected the gas furnace from the house wiring and plugged the furnace into the generator, so the fan and thermostat worked. We plugged satellite TV and lights into the generator as well. We had heat, television and light during the outage.